Systems and methods of operation of capacitive radio frequency micro-electromechanical switches

ABSTRACT

Disclosed are systems and methods of operation for capacitive radio frequency micro-electromechanical switches, such as CMUT cells for use in an ultrasound system. An RFMEMS may include substrate, a first electrode connected to the substrate, a membrane and a second electrode connected to the membrane. In some examples, there is a dielectric stack between the first and second electrodes. The dielectric stack design minimizes drift in the membrane collapse voltage. In other examples, a voltage supply coupled to an ultrasonic array compressing the CMUT cells is adapted to provide a sequence of voltage profiles to the electrodes of the CMUT cell, wherein each profile includes a bias voltage and a stimulus voltage, and wherein a polarity of each subsequent voltage profile in the sequence is opposite to the polarity of the preceding profile. In another example, there is a capacitance sensing circuit provided, which is arranged to determine a drift voltage of the CMUT cell.

RELATED APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2017/084088, filed on Dec.21, 2017, which claims the benefit of European Application No.16206309.3, filed Dec. 22, 2016. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to capacitive radio frequencymicro-electromechanical switches, RFMEMS, and in particular tocapacitive machined ultrasound transducers, CMUTs, for use in anultrasound imaging system.

The present invention further relates to a method of operating saidcapacitive RFMEMS and CMUTs.

BACKGROUND OF THE INVENTION

Ultrasonic transducers used for medical imaging have numerouscharacteristics that lead to the production of high quality diagnosticimages. Among these are broad bandwidth, enabling high resolution andhigh sensitivity, and large pressure output, enabling large depth offield of acoustic signals at ultrasonic frequencies. Conventionally thepiezoelectric materials which possess these characteristics have beenmade of PZT and PVDF materials, with PZT being particularly popular asthe material of choice. However, PZT suffers from a number of notabledrawbacks.

Firstly, ceramic PZT materials require manufacturing processes includingdicing, matching layer bonding, fillers, electroplating andinterconnections that are distinctly different and complex and requireextensive handling, all of which can result in transducer stack unityields that are lower than desired. This manufacturing complexityincreases the cost of the final transducer probe and puts designlimitations on the minimum spacing between the elements as well as thesize of the individual elements.

Moreover, PZT materials have poorly matched impedance to water orbiological tissue, such that matching layers need to be added to the PZTmaterials in order to obtain the desired acoustic impedance matchingwith the medium of interest.

As ultrasound system mainframes have become smaller and dominated byfield programmable gate arrays (FPGAs) and software for much of thesignal processing functionality, the cost of system mainframes hasdropped with the size of the systems. Ultrasound systems are nowavailable in inexpensive portable, desktop and handheld form, forinstance for use as ultrasound diagnostic imaging systems or asultrasound therapeutic systems in which a particular (tissue) anomaly isablated using high-energy ultrasound pulses. As a result, the cost ofthe transducer probe is an ever-increasing percentage of the overallcost of the system, an increase which has been accelerated by the adventof higher element-count arrays used for 3D imaging in the case ofultrasound diagnostic imaging systems.

The probes used for ultrasound 3D imaging with electronic steering relyon specialized semiconductor devices application-specific integratedcircuits (ASICs) which perform microbeam forming for two-dimensional(2D) arrays of transducer elements. Accordingly it is desirable to beable to manufacture transducer arrays with improved yields and at lowercost to facilitate the need for low-cost ultrasound systems, andpreferably by manufacturing processes compatible with semiconductorproduction.

Recent developments have led to the prospect that medical ultrasoundtransducers can be batch manufactured by semiconductor processes.Desirably these processes should be the same ones used to produce theASIC circuitry needed by an ultrasound probe such as a CMOS process.These developments have produced micromachined ultrasonic transducers orMUTs, the preferred form being the capacitive MUT (CMUT). CMUTtransducers are tiny diaphragm-like devices with electrodes that convertthe sound vibration of a received ultrasound signal into a modulatedcapacitance.

For transmission, the capacitive charge applied to the electrodes ismodulated to vibrate/move the diaphragm of the device and therebytransmit an ultrasound wave. Since these diaphragms are manufactured bysemiconductor processes the devices generally can have dimensions in the10-500 micrometer range, with the diaphragm diameter for instance beingselected to match the diaphragm diameter to the desired resonancefrequency (range) of the diaphragm, with spacing between the individualdiaphragms less than a few micrometers. Many such individual CMUT cellscan be connected together and operated in unison as a single transducerelement. For example, four to sixteen CMUT cells can be coupled togetherto function in unison as a single transducer element. A typical 2Dtransducer array can have 2000-10000 CMUT transducer elements or cellsby way of example.

The manufacture of CMUT transducer-based ultrasound systems is thereforemore cost-effective compared to PZT-based systems. Moreover, due to thematerials used in such semiconductor processes, the CMUT transducersexhibit much improved acoustic impedance matching to water andbiological tissue, which obviates the need for (multiple) matchinglayers and yields an improved effective bandwidth.

In order to optimize the acoustic power (output pressure) produced bythe CMUT cells, the CMUT cells may be operated in so-called collapsemode in which the CMUT cells are driven by a bias voltage that drives acentral part of the diaphragm or flexible membrane across the gap ontothe opposing substrate and provided with a stimulus having a setfrequency that causes the diaphragm or flexible membrane to resonate atthe set frequency. The voltage at which the membrane goes into collapseis called the collapse voltage, V_(c). However, a drawback of operatingCMUT cells in a collapse mode is that it negatively affects the lifetimeof the CMUT cells. This is largely caused by charging effects, namelypolarization, charge injection and space charge orientation, which occurin the dielectric layers that separate the electrodes in the CMUT cellsin the presence of the high electric field caused by the collapsevoltage. A further effect of this is the shifting of the collapsevoltage V_(c), over time. CMUT cells have operational ranges of biasvoltage and cell capacitance that define an operational window for thecell. A shift in bias voltage or cell capacitance lead to a shift in thetransmission and reception characteristics of the CMUT cells, resultingin a negative impact on the ultrasound image quality.

A CMUT cell essentially functions as a capacitive RFMEMS switch. Theissues described above, relating to drift to the CMUT cellcharacteristics, apply more generally to MEMS switches in particularcapacitive RFMEMS switches which are based on a resonant mode ofoperation.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention,there is provided an ultrasound system comprising:

an ultrasonic array of CMUT cells, each CMUT cell comprising asubstrate, a first electrode coupled to the substrate, a flexiblemembrane at least partially spatially separated from the firstelectrode, and a second electrode coupled to the flexible membrane;

a voltage supply coupled to the array, wherein the voltage supply isadapted to: provide a bias voltage and a stimulus voltage to theelectrodes of each CMUT cell; and

a capacitance sensing circuit adapted to derive an indication ofcapacitance of the CMUT and to determine an indication of drift voltageof the CMUT cell based on the indication of capacitance.

By providing an ultrasonic system with an array of CMUT cells, eachadapted to reduce or eliminate charging effects within themselves, thelifetime of the ultrasound system and the image quality produced by thesystem are increased.

The charging of a CMUT cell produces a shift in the collapse voltage,known as drift voltage, between the electrodes of the cell, leading to achange in the cell's capacitance. This leads to a change in thetransducer properties of the cell when operated at a constant voltage,as the transducer operation depends on both the driving voltage and thecell's capacitance. By providing the ultrasound system with acapacitance sensing circuit, it is possible to monitor the capacitance,and so the charging effects, of each CMUT cell in the CMUT array. Thisenables the system to monitor the condition of each cell and provide anindication to the system of when the charging of a cell may adverselyaffect the system's performance.

More particularly, the capacitance sensing circuit is adapted todetermine an indication of drift voltage of the CMUT cell based on aderived indication of the capacitance (or impedance). This may be aquantitative measure of drift voltage in some examples.

The capacitance sensing circuit may in particular be adapted todetermine a drift voltage of the CMUT cell based on determining a changein the capacitance, i.e. based on a difference between the derivedindication of the capacitance and a previously measured indication ofthe capacitance. The system may hence be adapted to recurrently deriveindications of capacitance (for instance at regular intervals), and toderive indications of drift voltage based on changes in the derivedmeasures of capacitance over time.

The indication(s) of capacitance may be derived while the bias voltageis applied, and while the stimulus voltage is not applied. For instance,the stimulus voltage may comprise a unipolar RF pulse, where thisconsists of intermittently applied sets or sequences of unipolar voltagepulses, each applied at a given pulse frequency. The capacitance maythen be derived between applied sets of pulses, while no stimulusvoltage is being applied.

For the avoidance of doubt, drift voltage means a shift or change in thecollapse voltage of the CMUT cell, the collapse voltage being thevoltage at which the CMUT cell enters collapse mode.

Collapse mode is an operating mode in which CMUT cells are driven by abias voltage that drives a central part of the diaphragm or flexiblemembrane across the gap onto the opposing substrate. A stimulus voltageis then applied that causes the diaphragm or flexible membrane toresonate at a given frequency. The voltage at which the membrane goesinto collapse is called the collapse voltage, V_(c).

In examples, drift voltage may be derived based on a known (e.g.pre-stored) relation between capacitance and drift voltage.

Drift voltage may be derived based on a known (e.g. pre-stored) relationbetween change in capacitance and drift voltage.

Drift voltage may be derived based on a known (e.g. pre-stored) relationbetween capacitance and collapse voltage. By measuring changes incapacitance, changes in the drift voltage can be ascertained.

In examples, drift voltage may be determined based on a knownrelationship between capacitance and applied bias voltage, in particularbased on a deviation from an expected capacitance, given a particularapplied bias voltage, based on this known relation. In particular, thereis an almost linear relation between capacitance and applied biasvoltage when the CMUT is in collapse mode. This relation may be stored,e.g. in a local memory such as an EPROM, and drift in collapse voltageascertained from a magnitude of deviation in capacitance compared towhat is expected for the applied bias based on the stored relation.

For the avoidance of doubt, in all embodiments, the bias voltage may bea DC voltage. However, the bias voltage may in embodiments be varied inorder to counter drift in collapse voltage. Hence, in this sense it maynot strictly be a DC voltage, but rather a bias voltage with a magnitudewhich may change (although much more slowly than the stimulus voltage).

The stimulus voltage may in examples be an AC voltage. However, inadvantageous embodiments, it may consist instead of a unipolar RF pulse,namely intermittently applied sequences of voltage pulses. This will beexplained in greater detail below.

In summary, the drift voltage is a shift or change or variation in thebias voltage at which the membrane is caused to move across the gap orcavity of the CMUT cell onto the opposing first electrode.

In an embodiment, the capacitance sensing circuit is adapted to:

generate a test signal, wherein the test signal has a predeterminedvoltage;

measure an attenuated signal of the test signal, wherein the test signalis attenuated by the impedance of at least the CMUT cell;

determine an impedance of the CMUT cell based on the attenuated signaland the test signal; and

determine a drift voltage of the CMUT cell based on the determinedimpedance.

As the test signal travels through the system, it is attenuated by theimpedance of various components of the system such as: a low-noiseamplifier; a coaxial cable connecting the ultrasonic probe to theultrasound system; and a CMUT cell.

Using the known test signal and the measured attenuated signal, it ispossible to calculate the impedance of the circuit. As the impedance ofthe components such as the low-noise amplifier and the coaxial cable areknown, it is possible to extract the impedance of the CMUT cell.

The impedance of the CMUT cell depends on the capacitance of the cell,and so is dependent on the charging of the cell. In this way, thecapacitance sensing circuit enables the ultrasound system to monitor thedrift voltage, and so the level of charging, of a CMUT cell.

In an arrangement, responsive to the absolute value of the drift voltagebeing above a predetermined value, the voltage supply is further adaptedto reverse the polarity of the bias voltage.

By reversing the polarity of the bias voltage, the charge held by theCMUT cell will be dissipated. By performing this function regularly, theCMUT cell will be prevented from building an excessive amount of chargethat would lead to a negative impact of the ultrasound image quality andlifetime of the ultrasonic system.

In further or other arrangements, responsive to the absolute value ofthe drift voltage being above a predetermined value, the voltage supplyis further adapted to reverse the polarity of the stimulus voltage.

In this way it is possible to further dissipate the charge in the CMUTcell whilst also enabling the cell to transmit the ultrasonic waves tothe subject.

As noted above, the stimulus voltage may comprise a unipolar RF pulse,i.e. intermittently applied sets or sequences of voltage pulses all ofthe same polarity. By reversing the polarity of these sequences ofpulses, charge can be dissipated.

In some cases, the voltage supply is adapted to reverse the polarity inless than 1 microsecond.

Performing the voltage reversal in this timeframe prevents acousticartifacts in the final ultrasound image.

In accordance with examples, the ultrasound system may comprise anultrasonic probe, wherein the ultrasonic probe comprises the array ofCMUT cells.

According to examples in accordance with a further aspect of theinvention, there is provided a method for operating a CMUT cell, theCMUT cell comprising:

a substrate;

a first electrode connected to the substrate;

a flexible membrane, wherein the flexible membrane is spatiallyseparated from the single electrode; and

a second electrode connected to the flexible membrane;

the method comprising performing a sequence of ultrasound generationcycles, each cycle comprising:

providing a bias voltage to either of the electrodes of a CMUT cell,wherein the bias voltage drives the CMUT cell into collapsed mode;

providing a stimulus voltage to either of the electrodes of the CMUTcell, wherein the stimulus voltage causes a portion of the flexiblemembrane to vibrate at a predetermined frequency; and

removing the stimulus voltage, thereby enabling the CMUT cell to receiveincoming acoustic signals,

wherein the sequence comprises:

-   -   first cycles with a first polarity of the bias voltage and        second cycles with an opposite second polarity of the bias        voltage, or    -   third cycles with a first polarity of the stimulus voltage and        fourth cycles with an opposite second polarity of the stimulus        voltage.

A typical ultrasound transmission sequence will consist of atransmission setup step, a transmit step and a receive step. By usingopposite polarity cycles, drift of the collapse voltage is corrected sothat a more stable performance of the CMUT cell is obtained over time.

The sequence may comprise alternate first and second cycles (i.e. analternating polarity of a bias voltage profile) or alternate third andfourth cycles (i.e. an alternating polarity of a stimulus voltageprofile).

The alternating cycles may thus be implemented automatically. Instead,there may be feedback control. For example, each ultrasound generationcycle may comprises determining a drift voltage of the CMUT cell.

By determining a drift voltage of the CMUT cell in the transmissionsetup step, it is possible to provide a bias voltage that will preventor counteract the charging of the CMUT cell before the transmit stepbegins. In this way, the CMUT cell may be discharged during the normaloperation of the ultrasound system, meaning that there is no requirementfor a separate discharging step.

The bias voltage selected based on the determined voltage may vary inmagnitude and polarity during the transmission setup step.

The stimulus voltage is provided in order to cause the flexible membraneof the CMUT cell to vibrate at a predetermined frequency. In this way,an ultrasonic RF pulse is generated and sent into the subject, forexample a patient.

Following this, the stimulus voltage is removed, enabling to theflexible membrane to vibrate freely in response to the reflectedultrasound waves returning from the subject.

In an embodiment, the step of determining a drift voltage of the CMUTcell comprises:

generating a test signal, wherein the test signal has a predeterminedvoltage;

measuring an attenuated signal of the test signal, wherein the testsignal is attenuated by the impedance of at least the CMUT cell;

determining an impedance of the CMUT cell based on the attenuated signaland the test signal; and

determining a drift voltage of the CMUT cell based on the determinedimpedance.

In some embodiments, the method further comprises, responsive todetermining that the absolute value of the drift voltage is above apredetermined value, reversing the polarity of the bias voltage.

In further or other embodiments, the method further comprises,responsive to determining that the absolute value of the drift voltageis above a predetermined value, reversing the polarity of the stimulusvoltage.

According to examples, there is provided a capacitive radio frequencymicro-electromechanical switch, RFMEMS, comprising:

a substrate;

a first electrode connected to the substrate;

a flexible membrane, wherein the flexible membrane is spatiallyseparated from the first electrode;

a second electrode connected to the flexible membrane; and

a dielectric stack between the first and second electrodes, comprising:

-   -   a first dielectric layer, wherein the first dielectric layer has        a first density of electrically active defects; and    -   a second dielectric layer, wherein the second dielectric layer        has a second density of electrically active defects, lower than        the first.

The CMUT cells of the CMUT cell array outlined above may each be anRFMEMS according to this example.

For example, when a capacitive RFMEMS switch has a bias voltage appliedto the first electrode, an electric field will be generated between thefirst and second electrodes. If the bias voltage exceeds a collapsevoltage of the capacitive RFMEMS switch, the switch will operate in acollapse mode, such as is used for a CMUT cell within an ultrasoundsystem. The electric field density will be strongest within thecollapsed portion of the switch as the two electrodes are closest atthis point. The electric field leads to the charging of the first andsecond dielectric layers of the dielectric stack. The electric fieldcauses the dielectric layers to become polarized leading to a negativeshift in the collapse voltage of the switch. The degree of polarizationdepends of the electrically active defect density of the dielectriclayer. The shift in the collapse voltage is known as a drift voltage.Thus, a drift voltage, and measurement of a drift voltage refers to achange in the collapse voltage.

A further effect of the electric field is the orientation of spacecharges within the dielectric layers. Charge carriers within thedielectric layers, i.e. those within the conduction band of thedielectric layers, will orientate towards the electrodes of the switchgenerating a space charge across the dielectric layers. The orientationof the space charges leads to a positive drift voltage. This effectdominates in dielectrics with a low electrically active defect density.

In addition, charge injection occurs in both dielectric layers due tothe tunneling of charge carriers from the first and second electrodes.Charge injection results in a negative drift voltage, thereby adding tothe effect of the polarization of the dielectric layers. By providing afirst and second dielectric layer that generate a negative and positiveshift in collapse voltage respectively, it is possible for the two driftvoltages to cancel each other out. Put another way, the overall driftvoltage is minimized by the opposing charging effects of the first andsecond dielectric layers.

The dielectric stack may be connected to the first electrode (i.e. thesubstrate electrode), so that dielectric stack is spatially separatedfrom the flexible membrane.

In an embodiment, the first and second dielectric layers are constructedfrom the same material. By preparing the two layers in a differentmanner, it is possible for the same material to exhibit differentdielectric properties.

In an embodiment, the first and second dielectric layers comprisesilicon dioxide, SiO₂.

Silicon dioxide, SiO₂, is a commercially available dielectric material.The dielectric properties of silicon dioxide vary depending on themethod of preparation.

In some embodiments, the first dielectric layer is constructed usingatomic layer deposition, ALD, leading to a greater susceptibility topolarization effects under an electric field.

By manufacturing the first dielectric layer through the atomic layerdeposition of SiO₂, the first dielectric layer will show a higher degreeof polarization effects compared to the second layer, leading to thegeneration of a negative voltage drift.

In an arrangement, the second dielectric layer is constructed usingchemical vapor deposition, CVD, leading to a greater susceptibilityspace charge orientation.

By manufacturing the second dielectric layer through the chemical vapordeposition of SiO₂, the second dielectric layer will show a higher levelof space charge orientation compared to the first dielectric layer,resulting in the generation of a positive voltage drift.

In some arrangements, the second dielectric layer is thicker than thefirst dielectric layer, for example at least two times thicker forexample three times thicker.

When the first and second dielectric layers are manufactured through theALD and CVD of SiO₂ respectively, the negative voltage drift associatedwith the first dielectric layer can be considerably larger than thepositive voltage drift associated with the second dielectric layer. Byproviding a thicker second dielectric layer, it is possible tocompensate for this and further minimize the drift voltage.

In some embodiments, the first and second dielectric layers comprisealuminum dioxide, Al3O2, or hafnium (IV) oxide, HfO2. Al3O2 and HfO2 arefurther examples of commercially available dielectrics that may beprepared in first and second layers that exhibit differing dielectricproperties.

In some designs, the dielectric stack further comprises:

a third dielectric layer, wherein the third dielectric layer is selectedbased on the dielectric properties of the first and second dielectriclayers.

By providing a third dielectric layer based on the dielectric propertiesof the first and second dielectric layers, the minimization of thevoltage drift may be further optimized. This becomes more relevant whenthe spatial requirements of the switch limit the changes in thicknessthat can be made to the first and second dielectric layers.

In a yet further design, wherein the first and second dielectric layerscomprise SiO2, the third dielectric layer comprises aluminum oxide,Al2O3. In this way, the third dielectric layer does not need to beconstructed from the same material as the first and second dielectriclayers, meaning that the shift in the collapse voltage, i.e. the driftvoltage, can be minimized through an optimal combination of dielectricproperties.

As noted, in various embodiments, the capacitive RFMEMS is a capacitivemicro-machined ultrasound transducer, CMUT, cell.

A capacitive micro-machined ultrasound transducer cell is an example ofa capacitive RFMEMS that can be used in the ultrasonic probe of anultrasound system for generating ultrasonic radio frequency pulses.

According to examples, there is provided an ultrasound systemcomprising:

-   -   an ultrasonic probe, wherein the ultrasonic probe comprises:    -   an array of CMUT cells as discussed above;    -   a voltage supply coupled to the ultrasonic probe, wherein the        voltage supply is adapted to:        -   provide a bias voltage to the first electrode of a CMUT            cell, wherein the bias voltage is adapted to drive the CMUT            cell into a collapse mode; and        -   provide a stimulus voltage to the second electrode of the            CMUT cell.

By providing an ultrasonic probe with CMUT cells comprising dielectriclayers capable of minimizing the voltage drift associated with drivingthe cells in collapse mode, as required by an ultrasound system, thelifetime of the system and the image quality produced by the system areimproved.

By operating the CMUT cells in collapse mode, the pressure output,bandwidth and operation stability of the CMUT array are able to meet thestandards required for medical ultrasound imaging. Operating the CMUTcells in collapsed mode often leads to a shift in the collapse voltageof the cell, known as a drift voltage, which negatively affects thelifetime of and image quality produced by the ultrasound system;however, this is overcome by providing CMUT cells with dielectric layerscapable of minimizing said drift voltage.

In some designs, the stimulus voltage is adapted to vibrate the flexiblemembrane of the CMUT cell at a predetermined frequency.

In this way, it is possible to generate a radio frequency (RF) pulse.For an ultrasound system, this pulse can range from 20 kHz to severaltens of MHz.

In various arrangements, the second electrode is adapted to detectincoming vibrations.

After an RF pulse is emitted into a subject, for example a patient, theRF pulse will travel through the subject until it meets a barrier. At abarrier, the ultrasonic waves of the RF pulse will be partiallyreflected back towards the ultrasound system. The ultrasonic wave willthen impact the surface of the ultrasonic probe, housing the CMUT array,and cause the flexible membrane of a CMUT cell to vibrate.

In an arrangement, the system further comprises:

a signal processor, wherein the signal processor is adapted to generatedata based on the incoming vibrations detected by the second electrode.

The vibrations of the flexible membrane of a CMUT cell can be detectedby the second electrode. In other words, the vibration of the flexiblemembrane will generate an electrical signal in the second electrode.This electrical signal can then be interpreted by a signal processor andused to generate data for the construction of an ultrasound image.

According to examples, there is provided a method for operating acapacitive RFMEMS, the capacitive RFMEMS comprising:

a substrate;

a first electrode connected to the substrate;

a flexible membrane, wherein the flexible membrane is spatiallyseparated from the first electrode;

a second electrode connected to the flexible membrane; and

a dielectric stack between the first and second electrodes, comprising:

-   -   a first dielectric layer, wherein the first dielectric layer has        a first density of electrically active defects; and    -   a second dielectric layer, wherein the second dielectric layer        has a second density of electrically active defects, lower than        the first,

the method comprising:

providing a bias voltage to the either of the electrodes of thecapacitive RFMEMS, thereby creating an electric field between the firstand second electrode, wherein the bias voltage is adapted to drive thecapacitive RFMEMS into a collapse mode;

providing a stimulus voltage to either of the electrodes, therebyincreasing an electric field between the first and second electrode;

polarizing the first dielectric layer to a first degree of polarizationand the second dielectric layer to a second degree of polarization,lower than the first degree, thereby causing a negative drift in thebias voltage between the first and second electrodes; and

orienting space charges within the first dielectric layer to a firstlevel of orientation and within the second dielectric layer to a secondlevel of orientation, greater than the first level, thereby causing apositive drift in the bias voltage between the first and secondelectrodes, thereby minimizing the overall drift in bias voltage betweenthe first and second electrodes.

According to examples, there is provided a capacitive micro-machinedultrasonic transducer, CMUT, cell, the CMUT cell comprising:

-   -   a substrate;    -   a first electrode connected to the substrate formed around a        central axis;    -   a flexible membrane, wherein the flexible membrane is spatially        separated from the first electrode; and    -   a second electrode connected to the flexible membrane, wherein        the second electrode is concentric with the first electrode,    -   wherein one of the first and second electrodes comprises a ring,        and there is a third electrode which occupies a middle portion        of the ring such that the ring and third electrodes are        spatially separated.

When operated in collapsed mode, the collapsed region of the CMUT cellexperiences the largest electric field strength. The electric fielddensity is determined by dividing the voltage applied across the deviceby the thickness of the layers between the electrodes. These layers cancomprise the flexible membrane and part of the substrate and may furtherinclude one or more dielectric layers. The electric field willconcentrate in the layer having the lowest dielectric permittivity. Thismeans that, whilst the electric field may not be high enough to cause anegative effect on all of the layers, the concentration of the electricfield in the collapsed portion of the CMUT cell may be high enough tocause charging to occur in the layer with the lowest dielectricpermittivity.

By providing one of the electrodes in the shape of a ring, therebyremoving the second electrode from the collapsed portion of the CMUTcell, the electric field within the collapsed portion of cell isreduced. In this way, it is possible to reduce the charging effectscaused by the electric field required to operate the CMUT in collapsedmode.

In an embodiment, the CMUT cell further comprises a third electrodeconnected to the flexible membrane, wherein the third electrode occupiesa middle portion of the ring defined by the second electrode, such thatthe second and third electrodes are spatially separated.

In this way, the third electrode can be connected to the flexiblemembrane, so as to occupy the collapsed portion of the CMUT cell. Thisenables a greater control of the electric field within the collapsedportion, which may then lead to further reducing the charging effects inthe collapsed portion of the cell.

In some embodiments, the third electrode is electrically grounded.

By grounding the third electrode, the collapsed portion of the CMUT cellonly experiences an electric field density corresponding to the biasvoltage needed to drive the cell into collapse mode. In other words, thestimulus voltage required to generate an ultrasonic RF pulse no longercontributes to the electric field density in the collapsed portion. Inthis way, the charging of the collapsed portion, and therefore the driftvoltage, is minimized or even prevented entirely.

In various embodiments, the first electrode is connected to the flexiblemembrane and the second and third electrodes are connected to thesubstrate.

It is possible to achieve the same effects as described above in thisalternate arrangement.

In some designs, the CMUT cell further comprises a support, formed aboutthe central axis, connected between the substrate and the flexiblemembrane, wherein the first electrode is in the shape of a ring.

By providing a support in the central region of the CMUT cell, it ispossible to operate the cell in a pre-stressed mode, which offerssimilar benefits to operating the cell in collapse mode. In addition, byforming the first electrode into the shape of a ring, similar to thesecond electrode, the electric field density in the central region,containing the support, is limited, thereby reducing charging within theCMUT cell.

It is noted that the methods and apparatus features may be used alone orin combination. Thus, the dielectric stack design may be used with orwithout the concentric electrode layout. Similarly, the method ofdetermining the drift voltage may be applied to a design with or withoutthe dielectric stack design, and with or without the concentricelectrode design.

It shall be understood by a person skilled in the field that in thedescribed therein examples of implanting the invention the voltagesupply may be adapted to provide the bias voltage and the stimulusvoltage to either the same electrode (the first or the secondelectrodes) or to separate electrodes (to one of the electrodes the biasvoltage, for example, and to the other of the electrodes the stimulusvoltage).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in more detail and by way ofnon-limiting examples with reference to the accompanying drawings,wherein:

FIG. 1 schematically depicts a typical CMUT cell of an ultrasound systemoperable in a collapsed mode;

FIGS. 2a, 2b, 3a and 3b depict operating principles of such a CMUT cell;

FIG. 4 is a contour plot of the acoustic performance of such a CMUTcell;

FIGS. 5a and 5b depict visual explanations of space charge orientationand polarization, within dielectrics of a CMUT cell, respectively;

FIG. 6 depicts an embodiment of an RFMEMS;

FIG. 7 depicts another embodiment of an RFMEMS;

FIG. 8 schematically depicts a method of operating the RFMEMS of FIGS. 5and 6;

FIGS. 9a and 9b depict a CMUT cell, according to an embodiment, inrelaxed and collapsed mode respectively;

FIGS. 10a and 10b depict a CMUT cell, according to another embodiment,in relaxed and collapsed mode respectively;

FIG. 11 depicts a pre-stressed CMUT cell according to an embodiment;

FIGS. 12a and 12b depict a CMUT cell, according to yet anotherembodiment, in relaxed and collapsed mode respectively;

FIG. 13 schematically depicts an example embodiment of an ultrasounddiagnostic imaging system; and

FIG. 14 depicts an example of a capacitance sensing circuit;

FIG. 15 depicts a method of operating a CMUT cell;

FIG. 16 depicts an embodiment of the method of FIG. 15;

FIG. 17 depicts another embodiment of the method of FIG. 15;

FIG. 18 depicts two embodiments of a bias voltage profile;

FIG. 19 illustrates determination of drift voltage via change in arelationship between applied voltage and exhibited capacitance;

FIGS. 20-22 illustrate measurement of drift voltage and examplecompensatory responses to drift voltage through polarity and magnitudechanges of applied bias and stimulus voltages.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the Figures are merely schematic and arenot drawn to scale. It should also be understood that the same referencenumerals are used throughout the Figures to indicate the same or similarparts.

The invention provides are systems and methods of operation forcapacitive radio frequency micro-electromechanical switches, such asCMUT cells for use in an ultrasound system. An RFMEMS includessubstrate, a first electrode connected to the substrate, a membrane anda second electrode connected to the membrane.

In some examples, there is a dielectric stack between the first andsecond electrodes. The dielectric stack design minimizes drift in themembrane collapse voltage. In other examples, one of the electrodes isin the form of a ring, and a third electrode is provided to occupy thespace in the center of the ring. Alternatively, the first and secondelectrodes are both in the form of a ring and there is a support betweenthe electrodes inside the rings.

FIG. 1 shows an aspect of an ultrasound system according to embodimentsof the present invention, in which the system includes an ultrasoundprobe having a transducer array comprising CMUT cells 100. The CMUTcells 100 according to embodiments of the invention will be explained inmore detail with the aid of FIGS. 6, 7 and 9-12. As will be explained infurther detail below, such an ultrasound system may be an ultrasounddiagnostic imaging system or may be an ultrasound therapeutic system.

Such a CMUT cell 100 typically comprises a flexible membrane ordiaphragm 114 suspended above a silicon substrate 112 with a gap orcavity 118 there between. A first electrode 122 is located on the floorof the cell on the upper surface of the substrate 112 in this example. Asecond electrode 120 is located on the diaphragm 114 and moves with thediaphragm. In the example shown, the two electrodes are circular.

A dielectric (not shown) is provided on the substrate 112 and underneaththe top (second) electrode 120. These two dielectrics may be equal incomposition and thickness, but may be also asymmetric (differentmaterials and thicknesses).

Other realizations of the electrode 120 design can be considered, suchas electrode 120 may be embedded in the membrane 114 or it may bedeposited on the membrane 114 as an additional layer. In this example,the first electrode 122 is circularly configured and embedded in thesubstrate layer 112 by way of non-limiting example. Other suitablearrangements, e.g. other electrode shapes and other locations of thefirst electrode 122, e.g. on the substrate layer 112 such that the firstelectrode 122 is directly exposed to the gap 118 or separated from thegap 118 by an electrically insulating layer or film to prevent ashort-circuit between the second electrode 120 and the first electrode122. In addition, the membrane layer 114 is fixed relative to the topface of the substrate layer 112 and configured and dimensioned so as todefine a spherical or cylindrical cavity 118 between the membrane layer114 and the substrate layer 112. It is noted for the avoidance of doubtthat in FIG. 1 the first electrode 122 is grounded by way ofnon-limiting example. Other arrangements, e.g. a grounded secondelectrode 120 or both second electrode 120 and first electrode 122floating are of course equally feasible.

The cell 100 and its gap 118 may exhibit alternative geometries. Forexample, cavity 118 could exhibit a rectangular or square cross-section,a hexagonal cross-section, an elliptical cross-section, or an irregularcross-section. Herein, reference to the diameter of the CMUT cell 100shall be understood as the biggest lateral dimension of the cell.

In FIG. 1, the diameter of the cylindrical cavity 118 is larger than thediameter of the circularly configured electrode plate 122. Electrode 120may have the same outer diameter as the circularly configured electrodeplate 122, although such conformance is not required and FIG. 1 shows alarger electrode plate 122. Thus, the second electrode 120 may be fixedrelative to the top face of the membrane layer 114 so as to align withthe first electrode plate 122 below. The electrodes of the CMUT cell 100provide the capacitive plates of the device and the gap 118 is thedielectric between the plates of the capacitor. When the diaphragmvibrates, the changing dimension of the dielectric gap between theplates provides a changing capacitance which is sensed as the responseof the CMUT cell 100 to a received acoustic echo.

The spacing between the electrodes is controlled by applying a staticvoltage, e.g. a DC bias voltage, to the electrodes with a voltage supply101. The voltage supply 101 may optionally comprise separate stages 102,104 for providing the DC and AC or stimulus components respectively ofthe drive voltage of the CMUT cells 100, e.g. in transmission mode. Thefirst stage 102 may be adapted to generate the static (DC) voltagecomponent and the second stage 104 may be adapted to generate analternating variable voltage component or stimulus having a setalternating frequency, which signal typically is the difference betweenthe overall drive voltage and the aforementioned static componentthereof. The static or bias component of the applied drive voltagepreferably meets or exceeds the threshold voltage for forcing the CMUTcells 100 into their collapsed states. This has the advantage that thefirst stage 102 may include relatively large capacitors, e.g. smoothingcapacitors, in order to generate a particularly low-noise staticcomponent of the overall voltage, which static component typicallydominates the overall voltage such that the noise characteristics of theoverall voltage signal will be dominated by the noise characteristics ofthis static component. Other suitable embodiments of the voltage sourcesupply 101 should be apparent, such as for instance an embodiment inwhich the voltage source supply 101 contains three discrete stagesincluding a first stage for generating the static DC component of theCMUT drive voltage, a second stage for generating the variable DCcomponent of the drive voltage and a third stage for generating thefrequency modulation or stimulus component of the signal, e.g. a pulsecircuit or the like. It is summarized that the voltage source supply 101may be implemented in any suitable manner.

It is known that by applying a static voltage above a certain threshold,the CMUT cell 100 is forced into a collapsed state in which the membrane114 collapses onto the substrate 112. This threshold value may depend onthe exact design of the CMUT cell 100 and is defined as the DC biasvoltage, known as the collapse voltage, at which the membrane 114 sticksto (contacts) the cell floor through the force due to the electric fieldbetween the electrodes. The amount (area) of contact between themembrane 114 and the substrate 112 is dependent on the applied biasvoltage. Increasing the contact area between the membrane 114 and thesubstrate 112 increases the resonant frequency of the membrane 114, aswill be explained in more detail with the aid of FIG. 2a and FIG. 3 a.

The frequency response of a collapsed mode CMUT cell 100 may be variedby adjusting the DC bias voltage applied to the CMUT electrodes aftercollapse. As a result, the resonant frequency of the CMUT cell increasesas a higher DC bias voltage is applied to the electrodes.

The principles behind this phenomenon are illustrated in FIGS. 2a, 2b,3a and 3b . The cross-sectional views of FIGS. 2a and 3a illustrate thisone-dimensionally by the distances D1 and D2 between the outer supportof the membrane 114 and the point where the membrane begins to touch thefloor of the cavity 118 in each illustration. It can be seen that thedistance D1 is a relatively long distance in FIG. 2a when a relativelylow bias voltage is applied, whereas the distance D2 in FIG. 3a is amuch shorter distance due to a higher bias voltage being applied. Thesedistances can be compared to long and short strings which are held bythe ends and then plucked. The long, relaxed string will vibrate at amuch lower frequency when plucked than will the shorter, tighter string.Analogously, the resonant frequency of the CMUT cell in FIG. 2a will belower than the resonant frequency of the CMUT cell in FIG. 3a which issubject to the higher bias voltage.

The phenomenon can also be appreciated from the two-dimensionalillustrations of FIGS. 2b and 3b , which vary as a function of theeffective operating area of the CMUT membrane. When the membrane 114just touches the floor of the CMUT cell as shown in FIG. 2a , theeffective vibrating area A1 of the non-contacting (free vibrating)portion of the cell membrane 114 is large as shown in FIG. 2b . Thesmall area 115 in the center represents the center contact region of themembrane. The large area membrane will vibrate at a relatively lowfrequency. This area 115 is an area of the membrane 114, which iscollapsed to the floor of the CMUT cell. When the membrane is pulledinto deeper collapse by a higher bias voltage as in FIG. 3a , the largercentral contact area 115′ results in a smaller free vibrating area A2 asshown in FIG. 3b . This lesser area A2 will vibrate at a higherfrequency than the larger A1 area. Thus, as the DC bias voltage isdecreased the frequency response of the collapsed CMUT cell decreases,and when the DC bias voltage increases the frequency response of thecollapsed CMUT cell increases.

FIG. 4 shows a contour plot of the acoustic pressure output of a typicalCMUT cell 100 in collapse mode as a function of applied DC bias voltageincluding a stimulus in the form of an AC modulation or frequencymodulation of constant frequency during transmission. The reciprocal ofthe corresponding pulse length is double the applied frequency. As canbe seen from this contour plot, when the CMUT cell 100 is operated at afixed or static voltage, e.g. a DC bias voltage of static value, optimalacoustic performance is obtained for a small range of frequencies only.However, when varying the bias voltage and the frequency modulation onthe bias voltage signal in a correlated manner, as indicated by thedashed line in the contour plot, the optimal acoustic performance of theCMUT cell 100 may be achieved over a much larger frequency range,thereby increasing the effective bandwidth of the ultrasound pulse (orpulse train) generated in the transmission mode of the ultrasound probeincluding the CMUT cell 100.

This can be understood in back reference to FIGS. 2a and 3a , whichexplained that the resonance frequency of the CMUT cell 100 in acollapsed state is a function of the applied (DC) bias voltage. Byadjusting the applied bias voltage when generating ultrasonic pulses ofa particular set frequency by applying a stimulus having the appropriateset frequency, pulses of different frequencies can be generatedexhibiting (near-) optimal acoustic performance of the CMUT cell 100 foreach pulse frequency. This therefore ensures (near-) optimal imagingresolution over a large bandwidth of the imaging spectrum.

FIGS. 5a and 5b show the effects of space charge orientation andpolarization, respectively, on the bias voltage of a CMUT cell.

FIG. 5a depicts a typical CMUT cell with a first electrode 122, a firstdielectric 150, a second electrode 120 and a second dielectric 155. Thefirst electrode 122 is electrically grounded and a positive biasvoltage, V_(b), is applied to the second electrode 120. A graph 160 ofvoltage against position within the CMUT cell depicts a first voltageprofile 165 across the CMUT cell when the bias voltage is first applied.The application of the bias voltage establishes an electric field 166between the first and second electrodes. The electric field strengthtypically lies in the range of 3.5MV/cm to 6MV/cm.

Charge carriers, such as electrons and holes, within the conductionbands of the first and second dielectrics move in response to theelectric field. In this example, the second electrode is positive,meaning that the electrons in the first 150 and second 155 dielectricwill gravitate towards the second electrode 120. In a similar manner,the positive charge of the second electrode 120 will repel positivecharge carriers in the first and second dielectric, resulting in acollection of positive charges near the first electrode 122. Thedistribution of these charges across the dielectrics is referred to asspace charge. In this case, the orientation of the space charge producesan electric field that acts to oppose the electric field 166 generatedby the first and second electrodes. This leads to a positive shift inthe voltage profile across the cell, which in turn leads to a reducedelectric field 171, having a reduced electric field strength compared toelectric field 166. As the electric field strength between theelectrodes is proportional to the voltage and distance between theelectrodes, the gradients of the voltage profiles 165 and 170 can beused as an indication of the change in electric field strength.

In this manner, the orientation of the space charges within the CMUTcell leads to a positive shift in the collapse voltage Vc (i.e. apositive drift). Since the electrical field in the gap is reduced, alarger voltage is needed to bring the membrane into collapse. In otherwords, at constant bias voltage the membrane would drift out ofcollapse.

FIG. 5b depicts the dielectric polarization that occurs in the same CMUTcell as depicted in FIG. 5a under the same bias voltage, V_(b). Thegraph 180 depicts the voltage profile 165 across the cell generated bythe application of the bias voltage. Once again, the bias voltageresults in the generation of an electric field 166.

The molecules that make up the dielectric layer behave as electricdipoles 185, each with an associated dipole moment. In the absence of anexternal electric field, the electric dipole moments of all of themolecules will be randomly aligned. When an external electric field 166is applied to the dielectrics, the electrical dipole moments of themolecules will align with the electric field, as shown in FIG. 5b . Thisleads to a reduced electric field 186 within the dielectric layers andan enlarged electric field 187 between the electrodes. This isrepresented as a voltage profile 190 on the graph 180. The polarizationof the dielectric layers due to the electric field leads to a negativeshift of the collapse voltage Vc across the cell. As the electricalfield is increased in the gap by polarization of the dielectrics, alower voltage is needed to bring the membrane into collapse. In otherwords: at contact bias voltage, the membrane would go deeper intocollapse due to the increased field in the gap.

This negative voltage drift is further reinforced by the injection ofcharge carriers 195 and 196 into the dielectric layers from theelectrodes by tunneling.

FIG. 6 shows a capacitive radio frequency micro-electromechanical switch100′, RFMEMS, according to an aspect of the invention. It may comprise aCMUT cell or another type of capacitive MEMS, such as capacitiveswitches used in pressure sensors and microphones. To illustrate thesimilarities with the CMUT cell described above, the same referencenumbers are used for corresponding components.

The switch includes a substrate 112 and a first electrode 122 connectedto said substrate. The first electrode may be connected to the topsurface of the substrate as depicted in FIG. 6; however, it may also bepositioned within the substrate or form a layer of the substrate itself.In addition, there is provided a dielectric stack 200 connected to thefirst electrode, such that the dielectric stack separates the firstelectrode and the substrate from the gap 118. The gap may be filled witha gas or it may be a partial vacuum.

The switch further comprises a flexible membrane 114 and a secondelectrode 120 connected to said membrane. The flexible membrane andsecond electrode are spatially separated from the dielectric stack,first electrode and substrate by the gap 118. FIG. 6 depicts the secondelectrode as being connected to the top surface of the flexiblemembrane; however, it may be provided within the membrane, form a layerof the membrane or be connected to the bottom surface of the membrane.

The dielectric stack comprises a first dielectric layer 210 and a seconddielectric layer 220. FIG. 6 depicts the first dielectric layer as beingconnected to the top surface of the second dielectric layer; however,the dielectric layer may also be reversed so that the second dielectriclayer is connected to the top surface of the first dielectric layer.

The first dielectric layer 210 is adapted to contain a first density ofelectrically active defects. The electrically active defects, also knownas traps, contribute to the strength of polarization effects that occurwithin a dielectric material under the influence of an electric field.The first dielectric layer is adapted to contain a density ofelectrically active defects that result in the polarization effectsdominating over the orientation of space charges. In this way, it ispossible for the first dielectric layer to result in a negative shift inthe collapse voltage of the capacitive RFMEMS.

In other words, the first dielectric layer 210 produces a negative driftvoltage.

The second dielectric layer 220 is adapted to contain a second densityof electrically active defects, lower than the first density within thefirst dielectric layer. The electrically active defect density of thesecond layer can lead to the orientation of space charges dominate overthe polarization effects under the influence of an electric field. Inthis way, the second dielectric layer produces a positive shift in thecollapse voltage of the capacitive RFMEMS.

In other words, the second dielectric layer 220 produces a positivedrift voltage.

Through the combination of the first dielectric layer 210 and the seconddielectric layer 220 in a dielectric stack 200, the negative driftvoltage and the positive drift voltage act to cancel each other out. Inthis way, the overall drift voltage produced by dielectric charging inthe switch may be reduced, improving both its lifetime and function. Thefirst and second dielectric layers may be constructed from the samematerial.

A commonly used dielectric material is silicon dioxide, SiO2, which maybe prepared in different ways in order to produce different dielectricproperties. The first dielectric layer may be constructed using atomiclayer deposition, ALD, and the second dielectric layer may beconstructed using chemical vapor deposition, CVD.

Atomic layer deposition is a thin film deposition method, wherein a filmof a given material is grown on a surface by exposing it to alternategaseous species. In the case of SiO2, atomic layer deposition leads to agreater number of electrically active defects in the bulk of thematerial, leading to a greater susceptibility to polarization effectsunder an electric field. In this way, the first dielectric layer 210will exhibit a negative drift voltage.

In a similar manner to ALD, CVD is the deposition of a desired materialon a surface when the surface is exposed to a volatile precursor gas. Inthe case of SiO2, a precursor gas of tetraethylorthoscilicate, TEOS, canbe used. Dielectric layers produced in this manner are less susceptibleto polarization effects than those produced by ALD, due to a reducednumber of electrically active defects. This allows the orientation ofspace charges to dominate the dielectric charging effects due to theelectric field. In this way, the second dielectric layer 220 willexhibit a positive drift voltage.

In this arrangement, polarization effects of the first dielectric layer210, manufactured from SiO2 using ALD, will produce a larger negativedrift voltage compared to the positive drift voltage produced by thesecond dielectric layer 220, manufactured from SiO2 using CVD. In orderto minimize the overall drift voltage of the capacitive RFMEMS 100, thesecond dielectric layer can be made thicker, in order to match themagnitude of the voltage drift of the first layer. In this case, thesecond dielectric layer is for example at least two times thicker, forexample three times thicker, than the first layer. In addition to therelative thickness between the dielectric layers, the absolute thicknessof the dielectric stack may be optimized in order to reduce the driftvoltage.

In cases where different materials, such as aluminum dioxide, Al3O2, orhafnium (IV) oxide, and/or manufacturing methods are used, the ratio ofthe thicknesses of the first and second layer may be altered in order tofurther optimize the minimization of the drift voltage.

FIG. 7 shows another embodiment of a capacitive RFMEMS 100′ according toan aspect of the invention. As shown in the figure, the dielectric stack200 can further comprise a third dielectric layer 215. The other layersare as described with referenced to FIG. 6. This layer 215 may beselected based on the dielectric properties of the first 210 and second220 dielectric layers. The dielectric properties of a material may beascertained through trap spectroscopy by charge injection and sensing,TSCIS, or leakage current spectroscopy (LCS). TCIS is sensitive to theinterface states of a material, wherein a lower number of interfacestates may serve as an indication that the material will produce apositive drift voltage. LCS is sensitive to the bulk states of amaterial. A higher number of bulk states in a material leads to largerpolarization effects under the influence of an electric field.

The inclusion of the third dielectric layer may allow the minimizationof the drift voltage to be further optimized. In the case where thefirst and second dielectric layers are both made from SiO2, the thirddielectric layer may be constructed from aluminum oxide, AL2O3.

The capacitive RFMEMS shown in FIGS. 6 and 7 may be a capacitivemicro-machined ultrasound transducer, CMUT, cell for use in anultrasound imaging system. A detailed description of a typicalultrasound imaging system is described further below in relation to FIG.13.

A basic ultrasound system may include an ultrasonic probe, comprising anarray of CMUT cells as described with reference to FIGS. 6 and 7, and avoltage supply adapted to provide a bias voltage to the first electrode122 and a stimulus voltage to the second electrode 120. In other cases,the bias voltage may be provided to the second electrode and thestimulus voltage provided to the first electrode.

The bias voltage may drive the CMUT cell into collapse mode, wherein theflexible membrane 114 contacts the dielectric stack 200, reducing thesize of the gap 118. This increases the electric field density in thecollapsed portion of the CMUT, leading to detrimental charging effects.By providing CMUT cell with a dielectric stack adapted to cancel out thedetrimental charging effects, the lifetime and performance of theultrasonic probe may be increased.

The stimulus voltage may cause the flexible membrane of the CMUT cell tovibrate at a predetermined frequency. In this way, the CMUT cell is ableto generate an ultrasonic pulse. This feature also applies in reverse,meaning that the flexible membrane may vibrate in response to incomingvibrations. The vibrations cause a change in capacitance of the CMUTcell which may be detected by the second electrode 120 in the form ofelectrical signals. These electrical signals may then be interpreted bya signal processor and used to generate image data for constructing anultrasound image.

FIG. 8 depicts a method 300 of operating a capacitive RFMEMS asdescribed with reference to FIGS. 6 and 7.

In step 310, a bias voltage is provided to the first electrode 120 of acapacitive RFMEMS. This bias voltage may be above a predetermined value,known as the collapse voltage, which drives the CMUT cell into acollapse mode. This bias voltage establishes an electric field betweenthe first and second electrodes.

In step 320, a stimulus voltage is provided to the second electrode inorder to produce an ultrasonic RF pulse by causing the non-collapsedportions of the flexible membrane to vibrate. This stimulus voltageincreases the electric field density within the switch, particularly inthe collapsed portion.

In step 330, the increased electric field causes the first dielectriclayer to become polarized to a first degree of polarization and thesecond dielectric layer to become polarized to a second degree, lowerthan the first degree. This generates a negative drift voltage.

In step 340, the first dielectric layer undergoes space chargeorientation to a first level and the second dielectric layer undergoesspace charge orientation to a second level, greater than the firstlevel. The orientations of the space charges across the two dielectriclayers, by the electric field, generate a positive drift voltage. Inthis way the overall drift voltage of the switch is reduced.

The dielectric stack arrangement has been described above in connectionwith a circular substrate electrode and a ring shaped membraneelectrode. However, this is only an example. The dielectric stack designmay be used with solid (i.e. not annular) electrodes as in the exampleof FIG. 1.

FIGS. 9a and 9b depict an embodiment of a CMUT cell 100, in relaxed andcollapse mode respectively. Again, layers which perform the samefunctions as in previous examples are given the same reference numbers.The CMUT cell 100 includes a substrate 112 and a first electrode 122connected to said substrate. The CMUT cell additionally includes aflexible membrane 114 and a second electrode 120, which are spatiallyseparated from the first electrode and substrate by a gap 118. Thesecond electrode is formed in the shape of a ring. The ring shape of thesecond electrode 120 is not limited to being purely circular, but maytake any shape where a central portion of the shape has been removed.

In other words, the second electrode 120 is shaped so that the electrodedoes not occupy a middle portion of the flexible membrane. Morespecifically, the second electrode is shaped so that the portion of theflexible membrane contacting the first electrode, when in collapse modeas shown in FIG. 9b , is not connected to the second electrode. FIG. 9bdepicts electric field lines 123 that describe the electric fieldgenerated when the first electrode is grounded and the bias voltage isapplied to the second electrode. The electric field density is highestbetween the electrodes, shown by the small spacing between the electricfield lines. By removing the central portion of the second electrode,the electric field density is reduced in the collapsed portion of theCMUT cell, as shown by the large spacing between the electric fieldlines, thereby reducing the level of charging effects, such asdielectric polarization, charge injection and space charge orientation.This results in a longer lifetime and improved performance of the CMUTcell.

The design of FIG. 9a does not have any dielectric stack. However asingle layer dielectric, or the two or three layer dielectric stackdescribed above may additionally be used as described further below withreference to FIG. 12. There may for example be a single or multiplelayer dielectric on the bottom electrode 122 and also a single ormultiple layer dielectric below the top electrode 120.

FIGS. 10a and 10b depict an embodiment of a CMUT cell, in relaxed andcollapse mode respectively, according to a further aspect of theinvention. The same layers as used in FIGS. 9a and 9b are given the samereference numbers and the description is not repeated. In this case, theCMUT cell includes a third electrode 124 occupying the middle portion ofthe second electrode. The third electrode may enable a greater level ofcontrol of the electric field density in the collapsed portion of theCMUT cell.

For example, the third electrode may be electrically grounded. In thisway, the electric field due to the stimulus voltage is removed, meaningthat only the electric field due to the bias voltage remains in thecollapsed potion of the CMUT cell. This further reduces the electricfield density in this portion of the cell, thereby further reducing thecharging effects within the cell.

The ring electrode and central (third) electrode may instead be formedon the substrate as the lower first electrode 122 and the upper membraneelectrode 120 may then be a continuous electrode.

Thus, one of the first and second electrodes comprises a ring, and thereis a third electrode which occupies a middle portion of the ring suchthat the ring and third electrodes are spatially separated.

FIG. 11 depicts another embodiment of a CMUT cell 100 according to afurther aspect of the invention. This embodiment shows a CMUT cellsimilar to the one depicted in FIGS. 9a and 9b with an additionalsupport 350 connected between the flexible membrane 114 and thesubstrate. In order to accommodate the support, the first electrode 122is made into the shape of a ring, wherein the support occupies themiddle portion of the ring. Thus, in this design both the first andsecond electrodes each comprise a ring, and the support 350 is formedabout the central axis, connected between the substrate and the flexiblemembrane.

The support 350 enables the CMUT cell to operate in a pre-stressed mode,which provides many of the benefits of operating the CMUT cell incollapsed mode with a reduced electric field density between theelectrodes. In this way, the charging effects of the CMUT cell arereduced.

FIGS. 12a and 12b depict yet another embodiment of a CMUT cell, inrelaxed and collapse mode, according to the further aspect of theinvention. According to this arrangement, a CMUT cell is provided with asecond electrode 120 in the shape of a ring and an electrically groundedthird electrode 124, as shown in FIGS. 10a and 10b , and a dielectricstack 200 containing a first dielectric layer 210 and a seconddielectric layer 220, as shown in FIG. 6.

In this design, an electrode configuration adapted to minimize thecharging effects of the electric field, when the CMUT cell is operatedin collapse mode, is combined with a dielectric stack adapted to cancelout any remaining charging effects. In this way it is possible tominimize, or eliminate, any charging effects, and therefore voltagedrift, in the CMUT cell, thereby further improving both the lifetime andperformance of the cell further.

In FIG. 13, an ultrasonic diagnostic imaging system with an arraytransducer probe 400 according to an example embodiment of the presentinvention is shown in block diagram form. In FIG. 13 a CMUT transducerarray 410, comprising CMUT cells as discussed above, is provided in anultrasound probe 400 for transmitting ultrasonic waves and receivingecho information. The transducer array 410 may be a one- or atwo-dimensional array of transducer elements capable of scanning in a 2Dplane or in three dimensions for 3D imaging.

The transducer array 410 is coupled to a microbeam former 412 in theprobe 410 which controls transmission and reception of signals by theCMUT array cells. Microbeam formers are capable of at least partial beamforming of the signals received by groups or “patches” of transducerelements for instance as described in U.S. Pat. No. 5,997,479 (Savord etal.), U.S. Pat. No. 6,013,032 (Savord), and U.S. Pat. No. 6,623,432(Powers et al.)

The microbeam former 412 is coupled by the probe cable, e.g. coaxialwire, to a transmit/receive (T/R) switch 416 which switches betweentransmission and reception modes and protects the main beam former 420from high energy transmit signals when a microbeam former is not presentor used and the transducer array 410 is operated directly by the mainsystem beam former 420. The transmission of ultrasonic beams from thetransducer array 410 under control of the microbeam former 412 isdirected by a transducer controller 418 coupled to the microbeam formerby the T/R switch 416 and the main system beam former 420, whichreceives input from the user's operation of the user interface orcontrol panel 438. One of the functions controlled by the transducercontroller 418 is the direction in which beams are steered and focused.Beams may be steered straight ahead from (orthogonal to) the transducerarray 410, or at different angles for a wider field of view. Thetransducer controller 418 may be coupled to control the aforementionedvoltage source 101 for the CMUT array. For instance, the voltage source101 sets the DC and AC bias voltage(s) that are applied to the CMUTcells of a CMUT array 410, e.g. to generate the ultrasonic RF pulses intransmission mode as explained above.

The partially beam-formed signals produced by the microbeam former 412are forwarded to the main beam former 420 where partially beam-formedsignals from individual patches of transducer elements are combined intoa fully beam-formed signal. For example, the main beam former 420 mayhave 128 channels, each of which receives a partially beam-formed signalfrom a patch of dozens or hundreds of CMUT transducer cells 100. In thisway the signals received by thousands of transducer elements of atransducer array 410 can contribute efficiently to a single beam-formedsignal.

The beam-formed signals are coupled to a signal processor 422. Thesignal processor 422 can process the received echo signals in variousways, such as bandpass filtering, decimation, I and Q componentseparation, and harmonic signal separation which acts to separate linearand nonlinear signals so as to enable the identification of nonlinear(higher harmonics of the fundamental frequency) echo signals returnedfrom tissue and microbubbles.

The signal processor 422 optionally may perform additional signalenhancement such as speckle reduction, signal compounding, and noiseelimination. The bandpass filter in the signal processor 422 may be atracking filter, with its passband sliding from a higher frequency bandto a lower frequency band as echo signals are received from increasingdepths, thereby rejecting the noise at higher frequencies from greaterdepths where these frequencies are devoid of anatomical information.

The processed signals are coupled to a B-mode processor 426 andoptionally to a Doppler processor 428. The B-mode processor 426 employsdetection of an amplitude of the received ultrasound signal for theimaging of structures in the body such as the tissue of organs andvessels in the body. B-mode images of structure of the body may beformed in either the harmonic image mode or the fundamental image modeor a combination of both for instance as described in U.S. Pat. No.6,283,919 (Roundhill et al.) and U.S. Pat. No. 6,458,083 (Jago et al.)

The Doppler processor 428, if present, processes temporally distinctsignals from tissue movement and blood flow for the detection of themotion of substances, such as the flow of blood cells in the imagefield. The Doppler processor typically includes a wall filter withparameters which may be set to pass and/or reject echoes returned fromselected types of materials in the body. For instance, the wall filtercan be set to have a passband characteristic which passes signal ofrelatively low amplitude from higher velocity materials while rejectingrelatively strong signals from lower or zero velocity material.

This passband characteristic will pass signals from flowing blood whilerejecting signals from nearby stationary or slowing moving objects suchas the wall of the heart. An inverse characteristic would pass signalsfrom moving tissue of the heart while rejecting blood flow signals forwhat is referred to as tissue Doppler imaging, detecting and depictingthe motion of tissue. The Doppler processor receives and processes asequence of temporally discrete echo signals from different points in animage field, the sequence of echoes from a particular point referred toas an ensemble. An ensemble of echoes received in rapid succession overa relatively short interval can be used to estimate the Doppler shiftfrequency of flowing blood, with the correspondence of the Dopplerfrequency to velocity indicating the blood flow velocity. An ensemble ofechoes received over a longer period of time is used to estimate thevelocity of slower flowing blood or slowly moving tissue.

The structural and motion signals produced by the B-mode (and Doppler)processor(s) are coupled to a scan converter 432 and a multiplanarreformatter 444. The scan converter 432 arranges the echo signals in thespatial relationship from which they were received in a desired imageformat. For instance, the scan converter may arrange the echo signalinto a two dimensional (2D) sector-shaped format, or a pyramidal threedimensional (3D) image.

The scan converter can overlay a B-mode structural image with colorscorresponding to motion at points in the image field with theirDoppler-estimated velocities to produce a color Doppler image whichdepicts the motion of tissue and blood flow in the image field. Themultiplanar reformatter 444 will convert echoes which are received frompoints in a common plane in a volumetric region of the body into anultrasonic image of that plane, for instance as described in U.S. Pat.No. 6,443,896 (Detmer). A volume renderer 442 converts the echo signalsof a 3D data set into a projected 3D image as viewed from a givenreference point as described in U.S. Pat. No. 6,530,885 (Entrekin etal.) The 2D or 3D images are coupled from the scan converter 432,multiplanar reformatter 444, and volume renderer 442 to an imageprocessor 430 for further enhancement, buffering and temporary storagefor display on an image display 440. In addition to being used forimaging, the blood flow values produced by the Doppler processor 428 andtissue structure information produced by the B-mode processor 426 arecoupled to a quantification processor 434. The quantification processorproduces measures of different flow conditions such as the volume rateof blood flow as well as structural measurements such as the sizes oforgans and gestational age. The quantification processor may receiveinput from the user control panel 438, such as the point in the anatomyof an image where a measurement is to be made.

Output data from the quantification processor is coupled to a graphicsprocessor 436 for the reproduction of measurement graphics and valueswith the image on the display 440. The graphics processor 436 can alsogenerate graphic overlays for display with the ultrasound images. Thesegraphic overlays can contain standard identifying information such aspatient name, date and time of the image, imaging parameters, and thelike. For these purposes the graphics processor receives input from theuser interface 438, such as patient name.

The user interface is also coupled to the transmit controller 418 tocontrol the generation of ultrasound signals from the transducer array410 and hence the images produced by the transducer array and theultrasound system. The user interface is also coupled to the multiplanarreformatter 444 for selection and control of the planes of multiplemultiplanar reformatted (MPR) images which may be used to performquantified measures in the image field of the MPR images.

As will be understood by the skilled person, the above embodiment of anultrasonic diagnostic imaging system is intended to give a non-limitingexample of such an ultrasonic diagnostic imaging system. The skilledperson will immediately realize that several variations in thearchitecture of the ultrasonic diagnostic imaging system are feasiblewithout departing from the teachings of the present invention. Forinstance, as also indicated in the above embodiment, the microbeamformer 412 and/or the Doppler processor 428 may be omitted, theultrasound probe 410 may not have 3D imaging capabilities and so on.Other variations will be apparent to the skilled person.

Moreover, it will be understood that the present invention is notlimited to an ultrasonic diagnostic imaging system. The teachings of thepresent invention are equally applicable to ultrasonic therapeuticsystems in which the CMUT cells 100 of the probe 400 may be operable intransmission mode only as there is no need to receive pulse echoes. Aswill be immediately apparent to the skilled person, in such therapeuticsystems the system components described with the aid of FIG. 12 andrequired to receive, process and display pulse echoes may be omittedwithout departing from the teachings of the present application.

According to the further aspect of the invention, the ultrasound systemfurther comprises a capacitance sensing circuit 441. FIG. 14 shows anembodiment of the capacitance sensing circuit. The capacitance sensingcircuit may be adapted to produce a test signal with a known voltage.For example, a generator 450 can be used to generate a small, sinusoidaltest signal, of a known voltage, V_(meassig), that propagates throughthe ultrasound system. The test signal is injected into the ultrasoundsystem by way of a large resistor, R_(meas). The resistance of R_(meas)may be for example 500Ω.

As the signal travels through the capacitance sensing circuit, it willundergo attenuation due to the impedance of the various components ofthe ultrasound system, such as a low noise amplifier (RX_LNA), a coaxialcable 451, and a CMUT cell (CMUT). The impedance of a component isproportional to the component's capacitance. The drift voltage of a CMUTcell, caused by the charging effects discussed above, leads to a changein the capacitance of the cell. By monitoring the capacitance of theCMUT cell, it is possible to monitor the level of charging induced bythe electric field within the cell. The impedance of a CMUT cell isgiven by the following formula:

${Z_{CMUT}} = \frac{1}{2\pi\; f \times C_{CMUT}}$

wherein: |Z_(CMUT)| is the magnitude of the impedance of the CMUT cell;f is the frequency of the test signal, which is selected from a range ofsignals appropriate for the operation of the CMUT cell and thecapacitance sensing circuit; and C_(CMUT) is the capacitance of the CMUTcell.

In an example where the test signal of the capacitance sensing circuitis attenuated by a low-noise amplifier (RX_LNA), a coaxial cable 451 andthe CMUT cell, the attenuated signal can be described using thefollowing formula:

$V_{meas} = {\frac{Z_{lna}/Z_{i\; n}}{R_{meas} + {Z_{lna}/Z_{i\; n}}}V_{meassig}}$

wherein: V_(meas) is the voltage (amplitude) of the attenuated signal;Z_(ina) is the impedance of the low-noise amplifier; Z_(in) is thecombined impedance of the coaxial cable and CMUT cell; R_(meas) is theresistance of the resistor used to inject the test signal into theultrasound system; and V_(meassig) is the voltage (amplitude) of thetest signal.

As the impedance of the coaxial cable is known from its length andcharacteristics, the impedance of the CMUT cell can be extracted fromthe value of Z_(in). In the example shown in FIG. 14, the length of thecoaxial cable is two meters, leading to an impedance of 50Ω. As shownabove, the value of the impedance of the CMUT cell can be used tocalculate the cell's capacitance. This then leads to a drift voltage ofthe CMUT cell, which indicates the level of charging occurring withinthe cell. The capacitance sensing circuit can be implemented for anentire array of the CMUT cells and thereby determining an average driftvoltage of the array, or per each CMUT cell in the array and therebydetermining a drift voltage of each individual cell. A capacitancesensing circuit can be also used for several sub-groups of the CMUTcells in the array.

If the capacitance sensing circuit determined that the absolute voltagedrift is above a predetermined value, selected to be a value where thecharging effects become detrimental to the function of the ultrasoundsystem such as 10V or 5V, the voltage supply 45 may be adapted toreverse the polarity of the bias voltage, the stimulus voltage, or both.By reversing the polarity of the voltages supplied to the CMUT cell, theelectric field generated between the electrodes is reversed. In thisway, the dielectric polarization and space charge orientations may bereduced, or eliminated. By performing the polarity reversal in less than1 microsecond, acoustic artifacts are avoided in the final image of theultrasound image produced by the system.

It shall be understood by the skilled in the art person that the FIG. 15depicts a method 500 of operating a CMUT cell.

In step 510, a drift voltage is determined for the CMUT cell. A methodfor performing this step is described with reference to FIG. 16.

In step 520, a bias voltage is provided to the first electrode of theCMUT cell, wherein the bias voltage is selected based on the determinedvoltage drift. The magnitude of the bias voltage is above the thresholdfor driving the CMUT cell into collapse mode. By selecting the biasvoltage based on the determined voltage drift, the charging effectsexperienced by the CMUT cell can be reduced or eliminated. For example,based on the determined voltage drift, the method may progress to step525, wherein the polarity of the bias voltage is reversed from theprevious cycle in order to reverse the electric field direction betweenthe electrodes of the cell. This may be done if it is determined, instep 510, that the magnitude of the voltage drift is above apredetermined value. This step is described in more detail further belowwith reference to FIGS. 17 and 18.

In step 530, a stimulus voltage may be provided to the second electrodeof the CMUT cell in order to cause the flexible membrane to vibrate at apredetermined frequency. In this way, an ultrasonic RF pulse may begenerated by the CMUT cell.

In a similar manner to above, if it is determined that the voltage driftis above a predetermined value, the polarity of the stimulus voltage maybe reversed in step 535. In this way, charging effects may be furtherreduced or eliminated. This step is described in more detail furtherbelow with reference to FIGS. 17 and 18.

In step 540, the stimulus voltage is removed in order to enable theflexible membrane to vibrate freely in response to incoming signals.These signals may be the reflected ultrasonic waves generated in step530. Following the reception period, the method returns to step 510 inorder to operate in a cyclical manner.

FIG. 16 depicts a method 600 for determining a voltage drift of a CMUTcell. In step 610, a test signal is generated at a predeterminedvoltage. This test signal may then be injected into the ultrasoundsystem.

In step 620, an attenuated signal is measured of the test signal. Theattenuated signal is attenuated by at least the impedance of the CMUTcell before it is measured. In step 630, the impedance of the CMUT cellis determined based on the test signal and the attenuated signal.

In step 640, a voltage drift of the CMUT cell is determined based on thedetermined impedance of the CMUT cell.

These steps are described in detail above with reference to thecapacitance sensing circuit 441 in FIGS. 13 and 14.

FIG. 17 depicts a method 700 for reversing the polarity of the biasvoltage (Vb) and stimulus voltage (RF driving). During the first imagingsequence 701, a bias voltage and stimulus voltage are applied to theCMUT cell in order to transmit an ultrasonic RF pulse. The CMUT cell isthen held in receive mode by the bias voltage as the stimulus voltage isremoved. Following the imaging sequence, the drift voltage of the CMUTcell is determined as described above with reference to FIG. 16. As thedrift voltage is dependent on the capacitance of the CMUT cell, a driftcapacitance (ΔC) may be determined. If ΔC is greater than a criticalvalue (C_(cr)) then the polarity of the bias voltage, and the stimulusvoltage, may be reversed for the second imaging sequence 702 in order toreduce the drift voltage of the CMUT cell. If ΔC is less than C_(cr),then the polarity may remain the same as in the first imaging sequenceas the drift voltage is not yet high enough to result in detrimentaleffects to the image quality of the ultrasound system. Thus, there is adetermination of whether the drift exceeds the threshold (Y=yes in FIG.17 and N=no).

FIG. 18 depicts two embodiments of a bias voltage profile that may beapplied to the first electrode of a CMUT cell in order to drive the cellinto collapse mode. The first bias voltage profile 720 illustrates amethod of applying a reduced bias voltage when the CMUT cell is notperforming an imaging sequence 721, 722. In this way it is possible toreduce the electric field density within the CMUT cell, thereby reducingthe charging effects. As the reduced bias voltage is above the collapsevoltage (V_(c+)), the CMUT cell remains in collapse mode, meaning thatit is prepared for the next imaging sequence.

The second bias voltage profile 730 depicts a similar method of reducingthe bias voltage between the first 731 and second 732 imaging sequences;however, in this case the polarity of the bias voltage is reversed inthe second imaging sequence 732 as described with reference to FIG. 17.In this way, the charging effects may be reduced by both the reducedbias voltage and the reversed polarity.

The switch to an opposite polarity of the bias voltage is performedrapidly for example within 1 microsecond.

It is also possible to implement the described scheme of reversedpolarity without a determination of the drift capacitance by means ofthe capacitance sensing circuit 441. As can be seen from the voltagebias profiles of the methods 700 and 730 the applied to the CMUT cellvoltage in opposite polarities can be symmetric with respect to zerovoltage. Thereby, the voltage supply can be arranged to alternateimaging sequences each having the same voltage amplitude profile and anopposite polarity such as 701 and 702. In other words, the voltagesupply coupled to the ultrasonic array is adapted to provide a sequenceof voltage profiles to the electrodes of the CMUT cell, wherein eachprofile includes a bias voltage and a stimulus voltage, and wherein apolarity of each subsequent voltage profile in the sequence is oppositeto the polarity of the preceding profile. When a variation in voltageprofile amplitude (irrespective of its polarity) remains the same foreach profile in the sequence, the charging effects on the CMUT cellmight be minimized due to the symmetry in the applied voltage profiles.It can be also beneficial for the ultrasonic system construction sincean additional circuitry can be omitted in this embodiment.

The change in polarity may take place at each subsequent transmissionevent. In this way, there is a symmetrical square wave bias voltage onwhich is superposed the transmission pulses. Thus, instead of thepolarity switching being dependent on a capacitance measurement as inFIG. 17, it may take place at each sequential transmission.

Thus, the concept of reversing polarity (of the bias voltage or thestimulus voltage, or both at different times) may be applied as anautomated sequence (once per transmission event, or even once every Ntransmission events) or it may be applied in an adaptive manner usingcapacitance sensing as a feedback control input.

As discussed above, systems and methods of the invention comprisedetermining an indication of drift voltage. This can be done indifferent ways.

As noted above, as change in capacitance is related to drift voltage, adrift capacitance ΔC can be determined and used as an indication ofdrift voltage.

Alternatively, drift voltage can be determined directly based on apre-determined, e.g. stored, relation between voltage and capacitance.This is illustrated in FIG. 19 which shows how a relation betweenvoltage and capacitance changes as a result of charging effects. Thisshift in the relationship is what causes the change in collapse voltage.The shift has the effect that for a given fixed applied bias voltage,the exhibited capacitance changes.

In FIG. 19, line 802 represents the original relationship betweencapacitance and voltage, while line 804 represents the shiftedrelationship. It can be seen that for a given applied bias voltageU_(bias), exhibited capacitance shifts by ΔC. If the originalrelationship is stored, it is possible to determine what voltage V_(cap)one would need to have applied based on the original relationship toobtain the new exhibited capacitance. From this, a voltage shift can bederived (V_(cap)−U_(bias)), and this gives an indication of the shift incollapse voltage (the drift voltage).

Alternatively, a direct relationship between measured change incapacitance and corresponding drift in collapse voltage may bepre-determined and stored and used to determine drift voltage directlybased on observed change in capacitance.

Capacitance and drift voltage are preferably determined while biasvoltage is applied, but while the stimulus voltage is not applied, e.g.between stimulus voltage pulses.

Examples are illustrated in FIGS. 20-22.

FIG. 20 shows applied voltage as a function of time. At t=0, both a biasvoltage 808 and stimulus voltage 806 are applied. The stimulus voltagetakes the form of a unipolar RF pulse, i.e. intermittently applied setsor sequences of voltage pulses all of the same polarity. Forillustration, a sequence of six unipolar pulses is shown in FIG. 20. Thecapacitance and drift voltage is preferably determined in-between thesesequences of pulses, while no stimulus voltage is being applied. Arrow812 indicates an example time point at which the determination could bemade.

As shown in FIG. 20, should the determined drift voltage exceed a giventhreshold, the polarity of one or both of the bias or stimulus voltagemay be reversed. In FIG. 20, the polarity of both the bias and driftvoltage is reversed. The reversed polarity stimulus voltage manifests ina sequence of negative voltage pulses.

FIG. 21 shows a further example. Again, a unipolar RF pulse stimulusvoltage 806 is applied in combination with a bias voltage 808. Driftvoltage may be determined after cessation of the stimulus voltage pulsesequence 806, e.g. at point 812. In this example, in response to driftvoltage above a given threshold, the bias voltage is reduced in stepwisefashion, ending at a reversed polarity voltage of equal and oppositemagnitude to the original bias voltage. The stimulus voltage polarity isalso reversed.

FIG. 22 shows a final example. Here the polarity of neither the stimulusvoltage 806 nor the bias voltage 808 is reversed. Rather, in response toa measured drift voltage (e.g. at point 812) exceeding a giventhreshold, the bias voltage is simply temporarily reduced in magnitude(a step-down in magnitude) before being stepped up again to the originalbias voltage. The stimulus voltage is then once again applied at thesame voltage polarity as before (i.e. a sequence of pulses is applied atthe same magnitude and polarity).

In accordance with one or more embodiments, drift voltage mayadditionally be obtained based on a measured pressure on the CMUT cellover the whole oscillatory cycle (pulse echo). The measured pressure isproportional to collapse voltage. Drift in collapse voltage cantherefore be determined based on a change in the measured pressure. Acorrection may then be applied in the case of a drift voltage exceedinga given threshold, e.g. by reversing the polarity of one or both of thebias and stimulus voltage, or by reducing the bias voltage temporarily.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention can be implemented by means of hardware comprising severaldistinct elements. In the device claim enumerating several means,several of these means can be embodied by one and the same item ofhardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention claimed is:
 1. An ultrasound system comprising: anultrasonic array of CMUT cells each CMUT cell comprising a substrate, afirst electrode coupled to the substrate, a flexible membrane at leastpartially spatially separated from the first electrode, and a secondelectrode coupled to the flexible membrane; and a voltage supply coupledto the ultrasonic array, wherein the voltage supply is adapted toprovide a sequence of voltage profiles to the electrodes of the CMUTcell, wherein each voltage profile includes a bias voltage and astimulus voltage, wherein a polarity of at least one of the bias voltageor the stimulus voltage is reversed based on a determined drift voltageof the CMUT cell.
 2. The ultrasound system according to claim 1, whereinthe voltage supply is further adapted to reverse the polarity of eachsubsequent stimulus voltage in less than 1 microsecond.
 3. Theultrasound system according to claim 1, wherein the second electrode ofeach CMUT cell is concentric with the first electrode; and wherein oneof the first and second electrodes comprises a ring, and there is athird electrode which occupies a middle portion of the ring such thatthe ring and third electrodes are spatially separated.
 4. The ultrasoundsystem according to claim 1, wherein the voltage supply is adapted to:provide the bias voltage to the first electrode; and provide thestimulus voltage to the second electrode of the CMUT cell.
 5. A methodfor operating a CMUT cell, the CMUT cell comprising: a substrate; afirst electrode connected to the substrate; a flexible membrane, whereinthe flexible membrane is spatially separated from the first electrode;and a second electrode connected to the flexible membrane; the methodcomprising performing a sequence of ultrasound generation cycles, eachultrasound generation cycle comprising: determining a drift voltage ofthe CMUT cell; providing a bias voltage to either of the electrodes ofthe CMUT cell, wherein the bias voltage drives the CMUT cell intocollapsed mode; providing a stimulus voltage to either of the electrodesof the CMUT cell, wherein the stimulus voltage causes a portion of theflexible membrane to vibrate at a predetermined frequency, wherein apolarity of at least one of the bias voltage or the stimulus voltage isreversed based on the determined drift voltage; and removing thestimulus voltage, thereby enabling the CMUT cell to receive incomingacoustic signals, wherein the sequence comprises first cycles with afirst polarity of the stimulus voltage and second cycles with anopposite second polarity of the stimulus voltage.
 6. The method asclaimed in claim 5, wherein the bias voltage is provided to the firstelectrode of a CMUT cell; and the stimulus voltage is provided to thesecond electrode of the CMUT cell.
 7. The method as claimed in claim 5,wherein the step of determining a drift voltage of the CMUT cellcomprises: generating a test signal, wherein the test signal has apredetermined voltage; measuring an attenuated signal of the testsignal, wherein the test signal is attenuated by the impedance of thecapacitance sensing circuit; determining an impedance of the CMUT cellbased on the attenuated signal and the test signal; and determining adrift voltage of the CMUT cell based on the determined impedance.
 8. Themethod as claimed in claim 5, wherein the method further comprises,responsive to determining that an absolute value of the drift voltage isabove a predetermined value, reversing the polarity of the bias voltage.9. The method as claimed in claim 5, wherein the method furthercomprises, responsive to determining that an absolute value of the driftvoltage is above a predetermined value, reversing the polarity of thestimulus voltage.
 10. The method as claimed in claim 5, wherein thesequence comprises third cycles with a first polarity of the biasvoltage and fourth cycles with an opposite second polarity of the biasvoltage.
 11. The method as claimed in claim 10, wherein the sequencecomprises alternate first and second cycles or alternate third andfourth cycles.